Martensitic steels are iron-base, steel alloys having a composition and given a heat treatment that produces a martensitic microstructure in the steel. The martensitic steels have wide application in the aircraft propulsion industry and are also used in other industries such as automotive. Metallic articles made of martensitic steels are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce molten metal, which is thereafter cast. Ore refinement may take place separately for each of the major alloying elements, or in combination for more than one element. Elements and combinations of elements may take many intermediate forms before being melted to form the final alloy. The metal is refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or during remelting. After a martensitic steel of the desired composition is produced, it may be used in the as-cast form for some compositions (i.e. cast martensitic steels), or it may be cast and further worked to form the metal to the desired shape for other compositions (i.e. wrought martensitic). It may instead be atomized to form fine powder and subsequently consolidated and, in some cases, further worked (i.e. powder metallurgy martensitic steels). In any case, further processing such as heat treating, machining, surface coating, and the like may be employed.
Regardless of the processing route, all of these forms involve melt processing and are, as a result, subject to restrictions imposed by such processes. Melting of martensitic steels is typically accomplished using multiple melt processes for premium-quality material in order to refine undesirable residual element content, to homogenize the overall composition, and to adjust the alloy content; or by single-melt processes and subsequent ladle modifications for standard-quality material. In either case, the melt is cast to produce the final ingot or article. In multiple melt processes, intermediate cast electrodes are produced which serve as the input stock to the subsequent melting step. Melting processes include those which are not performed under vacuum such as electroslag remelting, air melting, basic-oxygen-furnace melting, open-hearth-furnace melting, and electric-arc-furnace melting followed by argon-oxygen decarburization, and those which are performed under vacuum such as vacuum induction melting, vacuum arc remelting, and vacuum oxygen decarburization. Non-vacuum processes require the use of refining and protecting slags during the melting process. In any case, additional chemistry modifications may take place in the ladle to refine impurity content and add additional alloying elements. Additional limitations are also imposed as a result of the melting process for martensitic steels because of their composition. Alloy composition and resulting segregation issues during solidification impose practical limits on the melting rate and the maximum electrode or ingot diameters which can be produced without gross imperfections. In order to reduce the incidence of melt-related imperfections, strict melt controls are imposed so as to monitor and control melt rate, heat input, melt temperature, electrode-to-crucible clearance, and other critical parameters. Inadequate control of these parameters can result in material with significant solidification-related defects which, in turn, can reduce yield and increase production costs. In melting procedures, such as vacuum induction melting and other processes, which require vessels with refractory linings, where one alloy is melted in the ceramic melt crucible, and then a second alloy of a different composition is to be subsequently melted in the same vessel, an intermediate “wash heat” maybe required in order to minimize alloying element contamination from one alloy to the next which may result from residual alloy material remaining on the crucible wall. This wash heat requirement adds to the overall cost of producing high-quality martensitic steel.
Imperfections may result from melting processes or as a result of subsequent forming operations. Melt-related imperfections include those related to segregation as well as those resulting from extrinsic contaminants such as air and crucible ceramics. Melting of martensitic steels is subject to significant solidification segregation that can result in the formation of imperfections such as freckles, eutectic nodules, white spots, and banding. Freckles are the result of alloying-element partitioning during solidification, and are most prevalent in those materials that are highly alloyed to achieve improved properties. White spots, likewise, are a result of alloying-element segregation, but can also be associated with extrinsic contamination from crucible ceramics or remnant slag inclusions (dirty white spots). Inclusions such as sulfides and carbosulfides may also form as a result of high sulfur contents in the melts. Nitrides, alumina, and calcium aluminates may also form. These melt-related imperfections can significantly degrade the fatigue resistance and/or toughness of the martensitic steel. Melt-related imperfections can also contribute to forging-related imperfections such as cracking. Some highly alloyed materials are also more difficult to form as a result of inheriting the coarse cast structure which can lead to additional forging-related imperfections.
Some martensitic steels are also produced using powder metallurgy processes to circumvent these segregation imperfection issues, particularly for large-diameter ingots, and to reduce the size of extrinsic contaminants resulting from the multiple melt process. The current powder metallurgy processes, however, require martensitic steel to first be melted to produce alloy ingot, and then remelted and atomized to produce powder. These powder metallurgy processes add great expense and can still result in extrinsic contamination from crucible ceramics and slag. In addition, powder metallurgy processes are subject to concerns related to inert gas entrapment in powder particles during the atomization process, which can lead to residual porosity in the resulting billet or component. These imperfections can degrade the fatigue and/or toughness properties of articles produced by the current powder metallurgy process.
As a result, melting processes impose significant limitations on the resulting article. Incremental performance improvements resulting from processing modifications and incremental improvements in production cost reduction are still possible in a number of areas. However, the present inventors have recognized in the work leading to the present invention that in other instances the fabrication approach involving multiple melt steps imposes fundamental performance limitations that cannot be overcome at any reasonable cost. They have recognized a need for a departure from the conventional thinking in fabrication technology which will overcome many of these fundamental limitations. The present invention fulfils this need, and further provides related advantages.